The present invention relates to linear actuators and thrust force control mechanisms for such actuators.
Linear actuators are typically utilized wherever a thrust force is used for applying linear motion. Examples of the utilization of such a thrust force is in the operation of lever arms, cranks, slides and valve flaps. Such actuators are utilized for alternatively moving objects between predetermined limits. The actuator can be utilized for moving the movable member between positions within such predetermined limits by the utilization of appropriate feedback signals.
The typical linear actuator includes a drive motor which is connected to a drive screw through a pin type coupling. Rotation of the drive screw by the motor causes a mating drive nut and attached extension rod to move in an axial direction along a housing and to extend out of the housing. The object to which the thrust force is to be applied is coupled to the extension rod through a clevis mounted on the end of the extension rod.
When continuation of either the extension of the drive rod from the housing or withdrawal of the drive rod back into the housing is prevented, either due to completion of the stroke or by some external obstruction, an overload thrust protection system terminates operation of the drive motor. The overload thrust protection system utilized in accordance with the prior art is preset at the factory based on the arrangement of the various internal parts of the linear actuator. When the preset overload force has been obtained, an overload cam actuates a micro-switch which then interupts the power to the drive motor. An exemplary embodiment of such a linear actuator with an overload protection system is shown in U.S. Pat. No. 3,369,087 to Eller, the disclosure of which is incorporated herein by reference.
The prior art linear actuators do not allow for any adjustability of the thrust force limits to be selected at the site of operation of the linear actuators. In the above mentioned patent to Eller, the only possibility for adjustment of the thrust overload limits is by rearranging the position of the switches which requires disassembly of the machine. Alternatively, the requisite thrust force for overcoming the overload protection system can be modified by utilizing different springs for providing a greater or smaller force against the movement of the moveable actuating member; here again, disassembly and modification of the machine is necessary. In many situations, however, it is desirable for the thrust force limits to be selectively adjusted at the job site depending on the particular operation of the linear actuator. The prior art linear actuators, however, require disassembly of the actuator in order to provide any adjustability of the thrust overload limits. Hence, the switches that are provided are only utilized for terminating operation of the motor when maximum overloads have occured in accordance with the arrangement of the parts during manufacturing of the linear actuator.
In the operation of the prior linear actuators, if a breakdown in the switch should occur, there is no further protection against an overload force being applied to the springs so as to prevent total collapse of the springs and damage to them. Instead, total reliance is placed on the operation of the overload switches.
A further drawback of the prior linear actuators was the failure to fully protect the extension rods against lateral forces. Such lateral forces could cause malfunction of the actuator and if large enough then permanent damage to the actuator.